Vein grafts implanted in the arterial circulation undergo proliferative changes that often result in their occlusion. Most promienent among these changes are subintimal hyperplasia and fibrin deposition. Review of clinical data suggests that the above occur in inverse relationship to the flow velocity (V) rates and hence might be related to the wall shear stress (tau). Dogs develop lesions equivalent to those found in humans. Our experimental model exposes two equal segments of the same vein to different rates of flow velocity (V) and wall shear stress (tau) in the arterial circulation, while other parameters remain unchanged. This permits the correlation between the changes observed in the vein grafts and the rates of (V) and (tau) to which they are exposed. Scanning and transmission electron microscopy and light microscopy are used for evaluation of changes in the endothelium and wall of the vein. Flow velocity data are obtained from electromagnetic flow velocity transducers. Wall shear stress rates will be computed from geometry data and Fourier-analized velocity waveforms using the one- dimensional solution given by King.